[ac1982]

Allow as_filename(), as_path() and as_optional_path() to accept the existing cwd, in_app_dir and in_source_dir options. For example, config['path'].as_path(in_source_dir=True) can resolve a path relative to its YAML source without constructing a template manually.

The accessors forward keyword-only options to the existing templates, preserving resolution precedence, return types and optional-value handling. Document the options and retain mapping templates as the way to use relative_to, which requires sibling validation context.

Fixes #162.

Validation on Python 3.13/Linux:

• 11 initial regression cases fail before the change; final validation suite has 40 passing tests, including option precedence and updates.
• Full suite: 314 passed, 1 date-dependent release-helper failure reproduced on the unchanged base (Make release changelog test independent of the date #211).
• Strict Sphinx build, changed-doc formatting/lint and changed-Python Ruff checks pass.
• Full mypy and Ruff report only issues reproduced on the unchanged base (3 type errors and 1 lint error); full Python formatting passes.

AI assistance: implemented and tested with Codex.